In a dot matrix printer, a number of print wires are held in a print head in a fixed array. The print head is fixed to a carriage which typically moves within a limited range along a track through the successive printing positions. At each printing position, a predetermined number of print wires is actuated to strike the paper through an inking ribbon to form a portion of a dot matrix of a character on the paper. To actuate the print wires, electrical signals energize a predetermined number of electromagnets which control armatures which propel the wires toward the paper. Both the wires and their electromagnetic actuators are mounted in the print head, which typically has one or two columns of up to twelve wires per column facing the paper. Typically, the actuating ends of the print wires are arranged in a circular array about an axis of the print head. The actuators, including the armatures and the electromagnets, are positioned in a radial arrangement around the actuating ends of the print wires. The armatures extend radially outward from the print wires and the electromagnets are positioned near the outer end of the armature so as to actuate the armature in a generally axial direction and propel the print wire in the direction of the inking ribbon and the paper.
Wire matrix print heads of the above type have been shown and described in the prior art. U.S. Pat. No. 4,051,941 discloses a print head wherein an armature retainer is provided with a peripheral O-ring which bears against the outer ends of the armatures and causes them to pivot about the outermost edge of the electromagnet pole piece to a standby position. A second O-ring absorbs shock when the armature returns from the print position to the standby position. U.S. Pat. No. 4,407,591 discloses the use of pole pieces with generally triangular cross-sections. In addition, this patent discloses the use of a printed circuit board embedded in a resin block for making connections to the coils of the electromagnets.
A major objective in the design of wire matrix print heads is to improve the speed of operation. Since the device is electromechanical, it is inherently slower than the computer or other electronic device which sends data for printing. Therefore, the printer speed is a limiting factor in the operation of the computer system. Wire matrix print heads are usually rated in terms of the speed at which a single wire can be alternated between the standby position and the print position. In prior art print heads, the maximum rate has been on the order of 1,000-1,500 cycles per second. It is generally desirable to increase this speed so as to permit higher speed printer operation.
Another objective in the design of wire matrix print heads is to improve the ease of assembly and reduce the expense of manufacturing. Furthermore, it is desired to eliminate individual print wire adjustments and to provide a precision unit which will operate reliably for long periods.
It is a general object of the present invention to provide a new and improved wire matrix print head.
It is another object of the present invention to provide a wire matrix print head capable of extremely high speed operation.
It is another object of the present invention to provide a wire matrix print head with an electromagnetic actuator having a novel design capable of extremely high speed operation.
It is a further object of the present invention to provide a wire matrix print head with print wire paths carefully selected to minimize friction.